The document EP 2 270 478 A1 describes an optical sensor comprising an optical path, a light source for generating light and for feeding this light into the optical path, and a photo detector for detecting an optical signal coupled out of the first optical path. In addition, the optical path comprises, among other components, an optical resonator covered at least partially with an active layer of a covering material for selectively adsorbing molecules of a particular kind. A measurement implemented with this sensor makes use of the influence of adsorbed molecules on the optical length of the resonator, which causes resonance frequencies associated with a corresponding resonator to shift. In turn, this shift can be detected by observing the light emitted from the optical path. Thus, it is possible to detect whether a fluid which is brought into contact with the active layer contains molecules of the particular kind.
Even if very selective materials are used for the active layer, this layer will adsorb not only molecules of exactly one substance but rather molecules of a whole group of substances. This applies in particular for substances which are chemically similar. For this reason, ambiguities currently remain in the art as to whether the detected molecules are truly of a particular substance to be detected or only chemically similar to the particular substance.